help needed computing variance

Please help me solve this..Suppose n balls are distributed at random into r boxes. Let Xi=1 if box i is empty and let Xi=0 otherwise.

a)Compute E(Xi) – answer: (1-1/r)^n
b)for i not equal j compute E(Xi,Xj) -answer: (1-2/r)^n
c) Let Sr denote the number of empty boxes. Write Sr=X1+X2...and use result of a to compute E(Sr) -answer: r(1-1/r)^nd) Use the result of a and b and compute Var(Sr)?????I’m stuck on d… how do I compute var(Sr)???thanks

Please help me solve this..Suppose n balls are distributed at random into r boxes. Let Xi=1 if box i is empty and let Xi=0 otherwise.

a)Compute E(Xi) – answer: (1-1/r)^n
b)for i not equal j compute E(Xi,Xj) -answer: (1-2/r)^n
c) Let Sr denote the number of empty boxes. Write Sr=X1+X2...and use result of a to compute E(Sr) -answer: r(1-1/r)^nd) Use the result of a and b and compute Var(Sr)?????I’m stuck on d… how do I compute var(Sr)???thanks